I have a feeling that limit was set a long time ago before the warp speed drivers appeared on the market. Now it doesn't matter how gummy that 13-speed disc is, when it hits you it will do some damage. So I'm not really sure why there is still concern over how stiff discs are if they can still hurt someone.

Sure all discs hurt if you get hit.
I was hit by a putter (drive not putt) in the neck. And it all went black as I fell to the ground.

But I believe that the thing might be this.
If a disc is too stiff, there is a big risk it might shatter when thrown hard into a tree or something.

Throwing hard, stiff plastic could be like throwing cluster bombs.
(Not really but you get my point)